An electret is a material which has been imparted with an electrostatic charge internally by injecting an electric charge into an insulating polymer material. A piezoelectric sensor can be configured by laminating a signal electrode on a first surface of an electret sheet, and laminating a ground electrode on a second surface of the electret sheet.
PTL 1 discloses an electret sheet containing two kinds of polyolefins that are incompatible with each other, in which these two kinds of polyolefins form a phase-separated structure, and a polyolefin foam sheet formed as a result of the polyolefins being crosslinked by a polyfunctional monomer, is electrostatically charged by injection of a charge.